When Donald Trump, then a private citizen flirting with what most considered an improbable bid for the presidency, appeared on the Christian Broadcasting Network’s “The Brody File” in May 2015, he laid out his controversial solution to the United States’ problems with illegal immigration.

“I would build a wall like nobody builds a wall and nobody comes in illegally anymore,” the billionaire real estate developer said, adding, “I would have Mexico pay for it.”

Although Trump once tweeted, “We build too many walls and not enough bridges,” he had since mid-2014 been staunchly advocating the erection of a border wall.

Weeks after the Brody interview, Trump made his fateful descent down the escalator in Trump Tower and delivered the promise that would become a signature plank of his policy platform.

“I would build a great wall, and nobody builds walls better than me, believe me,” he said, “and I'll build them very inexpensively, I will build a great, great wall on our southern border. And I will have Mexico pay for that wall.”

Trump has demanded those funds be included in any deal that provides protection for those who qualified for the Obama-era Deferred Action for Childhood Arrivals program. Democrats want a clean vote on providing a path to citizenship for those young immigrants who were brought to the country illegally as children before they support a resolution to keep the government funded past Friday.

Republicans insist DACA recipients must be dealt with separately from government funding as part of broader immigration reform. Some conservative lawmakers say they have an obligation to the voters who gave their party control of the House, Senate, and White House to erect Trump’s wall and take a firm stance against illegal immigration.

“We should do what we campaigned on, which is build the wall, end chain migration, get rid of the diversity lottery, have e-verify, and stop sanctuary cities,” said Rep. Jim Jordan, R-Ohio, Wednesday. “Let’s do all those things first, then we can talk about the DACA issue.”

“If this was an election about DACA, Hillary Clinton would have won it,” Rep. Matt Gaetz, R-Fla., said. “Donald Trump won this election because it was about border security.”

Trump has already rejected one moderate compromise on immigration, and negotiations with Democrats stalled over vulgar comments he reportedly made about African and Haitian immigrants. Senate Majority Leader Mitch McConnell, R-Ky., has said it is unclear what Trump wants.

House Republicans are working to pass a continuing resolution to fund the government into February without addressing immigration. Senate Democrats believe they have the votes to block any spending bill that does not resolve the DACA issue.

Complicating matters further, White House Chief of Staff John Kelly told members of the Congressional Hispanic Caucus Wednesday that the views expressed by then-candidate Trump were “uninformed” and Mexico will never pay for a wall, according to the Washington Post.

“He has evolved in the way he looks at things,” Kelly later said on Fox News. “Campaign to governing are two different things, and this president has been very flexible in terms of what’s in the realm of the possible.”

Kelly’s comments reportedly infuriated Trump, leading to a pair of tweets early Thursday reasserting the stance that Kelly seems to have suggested was unwise and unrealistic.

“The Wall is the Wall, it has never changed or evolved from the first day I conceived of it,” Trump wrote, adding, “The Wall will be paid for, directly or indirectly, or through longer term reimbursement, by Mexico, which has a ridiculous $71 billion dollar trade surplus with the U.S.”

According to the U.S. Census Bureau, the trade deficit between the U.S. and Mexico in the first 11 months of 2017 was about $66 billion. Economists have questioned Trump’s conclusion that this figure, primarily the result of trading and investment by individuals and companies, has any bearing on the funding of a wall.

"It’s not like there is $54 billion sitting around somewhere in Mexico, like a magic pile of dollars, that could be used to build a wall," Mark Perry, a scholar at the American Enterprise Institute, told PolitiFact for a 2016 article that rated Trump’s claim that the trade imbalance means Mexico can afford to pay “false.”

In many aspects, Trump is correct that “The Wall is the Wall” as he has long described it.

Trump’s early statements and tweets were vague on exactly how long of a wall he envisioned, but after announcing his campaign, he appears to have settled pretty quickly on a barrier that would cover about 1,000 miles of the 2,000-mile border.

“I want nothing to do with Mexico other than to build an impenetrable WALL and stop them from ripping off U.S.,” he tweeted in March 2015.

At the first Republican primary debate in August 2015, Trump added a “big, beautiful door” to the wall for people to enter the country legally. At a town hall in New Hampshire later that month, he drew comparisons between what he hoped would someday be named the “Trump Wall” and the Great Wall of China.

"Well, 3,000 years ago, the Great Wall of China was built. We would like to have that wall. That wall, nobody gets through. That I can tell you. And that's 13,000 miles. right?" he said.

In October 2015, he revived the Great Wall analogy, but he made clear his wall would be only a fraction of the size.

“That's 13,000 miles,” he said. “Here, we actually need 1,000 because we have natural barriers. So we need 1,000. We can do a wall. We're going to have a big, fat beautiful door right in the middle of the wall.”

“You don’t need a wall where you have a natural barrier that’s far greater than any wall you could build, OK?,” he said. “Because somebody said oh, he’s going to make the wall smaller. I’m not going to make it smaller. The wall was always going to be a wall where we needed it.”

“People say, how will you get Mexico to pay? A politician other than the people in the states — I don't want to — a politician cannot get them to pay. I can,” he said at the October 2015 debate before again bringing up the trade imbalance.

In April 2016, the Trump campaign provided a memo to the Washington Post outlining a plan to cut off remittances immigrants in the U.S. send back to their families unless the Mexican government makes a one-time payment of $5-10 billion for the wall. Experts questioned the legality and feasibility of such a scheme, but the memo also suggested raising tariffs, cancelling visas, and increasing fees for border-crossing cards.

Trump has since wavered, acknowledging that the initial funding will have to come from U.S. taxpayers but insisting Mexico will directly or indirectly pay them back.

“Dishonest media says Mexico won't be paying for the wall if they pay a little later so the wall can be built more quickly,” he complained in a Jan. 8 2017 tweet. “Media is fake!”

In the recent WSJ interview, Trump suggested he could somehow take a percentage of money saved from renegotiating NAFTA to pay for the wall.

“Is it going to be 45 feet or 40 feet?” Trump said of the height at a March 2016 debate. “That could very well be…. But there's always give and take. There's always negotiation. And the best negotiator that knows what he's doing will make a great deal.”

For advocates of tougher immigration policies, Kelly’s description of Trump as having “evolved” on these issues is troubling.

“The bottom line is, these are the things he ran on,” said Ira Mehlman, media director for the Federation for American Immigration Reform. “These were centerpieces of his campaign. To suggest now that he was shooting from the hip, he didn’t know what he was talking about, it’s not plausible, it’s not acceptable.”

Tom Whalen, a professor of social science at Boston University and an expert on presidential history, said the open contradiction between a president and his chief of staff is unusual, and Trump may see it as a threat.

“Basically, Kelly is publicly calling out the president saying at the least, the president doesn’t have his facts straight, and at worst, he’s delusional,” he said.

It is at times difficult for lawmakers and activists to know who is speaking for this administration, but Mehlman emphasized that the goal of those seeking better border security has never been walling off all 2,000 miles.

“Physical barriers where physical barriers are practical and can make a difference…,” he said. “It isn’t one thing, it’s a combination. As important, maybe more important, are policies that make it clear to people there’s no reason to come across the border in the first place.”

Though he credited Trump’s tough immigration rhetoric for driving drastic reductions in illegal border crossings in 2017, Mehlman rejected the notion that those statistics undermine the need for the wall.

“Policies vary from administration to administration, and the next administration might take a very different view…. Once the physical barrier is there, it’s there and it will continue to have some beneficial effect,” he said.

According to Whalen, Trump may have painted himself into a rhetorical corner over the last three years, particularly with the “Mexico will pay” vows. If he fails to keep those promises or, as Kelly suggested, evolves because they were not possible, he risks an embarrassment reminiscent of President George H.W. Bush’s broken “no new taxes” pledge.

“It’s all important to his base,” Whalen said. “From day one, he has always catered to that core base that got him through the primaries and the general election, the so-called deplorables…. He thinks his presidency kind of hangs in the balance.”